The present invention relates to hazardous waste disposal systems, and more particularly to an improved incineration system and method which results in the efficient destruction of liquid and solid wastes in an apparatus including a primary incineration combustion means, at least one afterburner and a flue gas treatment system.
A typical waste incineration system for the destruction and removal of hazardous wastes consists of a primary incineration combustion apparatus, an afterburner and a flue gas treatment system. Additionally, the incineration system may include:
a solid and/or liquid waste feed system; PA1 a system for feeding an auxiliary fuel, usually in gaseous or liquid form; PA1 a system for feeding oxidizer, usually air and sometimes oxygen or an oxygen enriched air; PA1 a system for the evacuation of incombustible solid products of incineration, such as bottom ash; PA1 a system of heat recovery from the hot exhaust combustion flue gases with generation of preheated combustion air for waste incineration units, hot water, steam and/or electricity; PA1 a system for preparing, feeding, recycling and treating any water solutions produced for removal of gaseous and/or particulates in the flue gas treatment system; PA1 a stack for the discharge of treated flue gases to the atmosphere; PA1 a control system including flow, pressure and temperature transducers and controllers for controlling the flow of fuel and oxidizers, process temperatures and pressures at strategic locations in the system; and PA1 a flue gas sampling system.
The primary incineration combustion apparatus for solid and liquid wastes and sludges may be embodied as rotary kilns, multiple hearth furnaces, fluidized bed furnaces, grate furnaces and other combustion apparatus. Liquid and semiliquid pumpable wastes can also be combusted in cyclonic reactors as well as in various burners during the initial thermal destruction step of incineration process.
The rotary kiln is the preferable embodiment of the primary incineration process due to its versatility. It is arranged as a cylindrical refractory lined vessel rotating about a slightly inclined axis. The residence time in the kiln varies from a fraction of a second to several seconds for gaseous materials and from several minutes to several hours for solid materials. Solid wastes can be charged in a kiln either continuously as in the case of shredded material or as a batch charge as in the case of containerized materials such as drums or bundles. Special loading devices are used for charging solid wastes while pumpable liquid wastes and sludges are typically introduced directly into the kiln. The combustible fraction of wastes is partially pyrolysed and oxidized in the kiln. An auxiliary fuel such as combustible liquid waste, oil, natural gas or propane is commonly used for preheating the kiln lining, for providing supplemental heating while combusting low caloric value wastes, and for insuring the combustion stability.
Although the design of other primary incineration combustion units differs from that of a rotary kiln, they typically accomplish the same functions and contain many of the same functional elements as the rotary kilns and exhibit much the same disadvantages as those discussed below for the kilns.
Afterburners are typically cylindrical refractory lined vessels equipped with an auxiliary burner which is fed with a liquid and/or gaseous fuel and an oxidizer. Combustible liquid wastes can be used instead of, or in addition to, the auxiliary fuel. Afterburners are used to insure combustion of organic vapors, soot and other combustible components remaining after the primary incineration process. The afterburners provide a high temperature, highly oxidizing atmosphere with sufficient residence time and mixing of combustible vapors with oxygen to insure the required degree of organics destruction.
The most typical unit for treatment of flue gases leaving the afterburner is a wet scrubber wherein the combustion gases are washed by water or water solutions. Soot and halogens are largely absorbed and sulfur dioxide and nitrogen oxides are partially removed in the scrubber. Some polar organics and organics which are adsorbed in the soot are also partially removed. An alkali is often added to the scrubbing water to increase the efficiency of scrubbing of halogens and sulfur dioxide. Electrostatic precipitators or dust baghouses are often used for removal of the particulates from flue gases.
Heat recovery units are often installed between thermal destruction and flue gas treatment units. Heat of hot combustion flue gases may be used to preheat the combustion air for the primary incinerator and/or afterburner.
Solid and liquid wastes typically contain organic and inorganic combustible constituents. A fraction of organics may be highly toxic, mutanogenic and teratogenic. This fraction of organics is usually called principle organic hydrocarbons (POHC). Many POHCs are very stable and require oxidation at elevated temperatures for their destruction. When wastes are charged into a kiln, a rapid volatilization and partial pyrolysis of organics, including POHCs and water, if any, occurs. The volatilized components of organics require an adequate quantity of oxygen for their oxidation. Fuel and oxygen are also needed to supply heat for vaporization of water and organics and for raising the temperature to required levels.
The appropriate firing rate and combustion air feed rate are selected to provide adequate temperatures and excess oxygen level for the incineration system to achieve the required destruction efficiency of the POHCs for a given type and quantity of wastes. This temperature and excess oxygen level will be maintained by the control system. Other nonhazardous organics present as well as the fuel are usually essentially oxidized when POHCs are oxidized in the primary incineration combustion apparatus; however, new intermediate products may be formed during the combustion process. These products include carbon microparticles, carbon monoxide and an array of organic compounds. Many of these organic compounds are a higher molecular weight polycyclic or polyaromatic organics such as dioxins, benz(a) pyrene, dibenz(a,c)anthracene, picene, dibenz(a,h)anthracene, 7, 12-dimethyl(a)anthracene, benz(b)fluortane, 9,10-dimethylanthracene. These higher molecular weight organics are often called products of incomplete combustion (PICs). PICs are often as hazardous as POHCs. A fraction of PICs becomes absorbed on carbon microparticles. The combined PICs and carbon particles represent soot. Accordingly, soot is also a hazardous product. Carbon monoxide is also a toxic constituent and only a limited quantity of it may be permitted for discharge into the atmosphere. Therefore, the waste incineration steps must insure the thermal destruction of carbon monoxide, soot and PICs in the gaseous phase. Such destruction should be provided prior to the discharge of the combustion gases from the afterburner.
Both the feed rate and the properties of wastes which are fed into the combustion system may vary. Extreme variations in the feed rate occur during the so called batch charge when a substantial quantity of wastes is rammed or otherwise introduced into the apparatus in a short period of time. Gradual variations in the feed rate are also possible for continuously charged waste streams. The operational objective of an incineration system is to maximize the amount of waste passing through the system while minimizing the amounts of discharged flue gases, POHCs, and PICs. Generally, the maximum allowable concentrations of pollutants in the flue gases are specified in the operating permit which is based on the current environmental requirements and regulations.
In order to achieve this operational objective high temperatures, sufficient retention time and high turbulence should be provided in both the primary incineration combustion apparatus and the afterburner. Typically, the kiln temperature ranges from 750.degree. C. (1400.degree. F.) to above 1100.degree. C. (2500.degree. F.). The residence time for gases in both the kiln and the afterburner ranges from a fraction of a second to several seconds. Turbulence in either the kiln or the afterburner is not defined quantitatively, however. It is usually assumed that mixing is sufficient to heat adequately all elementary streams of gases and to provide a sufficient contact between organics and oxygen molecules in the furnace. In order to insure the sufficient contact between organics and oxygen, an excess of combustion air in the range of 5% to 200% of stoichiometric is commonly used.
Temperature, retention time, level of excess air and turbulence in the primary incineration combustion apparatus and afterburner effect the destruction efficiency which may be maintained during the operation of a conventional incineration system. An increase in any of these parameters will enhance the destruction efficiency. Attempts to improve destruction efficiency by increasing one or more of the above parameters, however, has not proven to be effective utilizing currently available incineration systems because of a corresponding drop in one of the parameters as one of the others is increased. For example, a higher level of excess oxygen provided by an increase in the air feed results in a lower temperature and lower retention time of gases in the furnace. An increase of the temperature by raising the amount of auxiliary fuel results in increase of combustion product volume which reduces retention time.
The incompatible nature of these parameters in existing incineration systems has limited the capability of existing incineration systems to dynamically intensify the incineration process to overcome transient process malfunctions leading to process failures. Typical transient malfunctions resulting in incineration process failure modes are described below using the kiln as an example for the primary incineration apparatus.
When wastes are charged in large batches or when loading rates of liquids and sludges are rapidly increased, the quantity of oxygen present in the kiln and the amount of oxygen being fed into the kiln during the rapid vaporization stage typically is not sufficient for complete combustion to occur, resulting in an overcharging failure. Only a fraction of combustible constituents of wastes, including POHC, is completely oxidized, forming CO.sub.2 and H.sub.2 O. The remaining organics are partially pyrolyzed and oxidized, thus forming carbon microparticles, CO and PICs. Vaporized fractions of POHCs and of wastes together with carbon microparticles, CO and PICs formed are transferred in an increased amount into the afterburner, so that afterburner is also overloaded. Meeting the oxygen requirements during the overload period in the kiln by substantially increasing the level of continuous combustion air feed rate would result in a shortening of the retention time for volatilized and partially pyrolyzed products in the kiln and may degrade the flame stability. This problem is aggravated by the fact that the substantially excessive air feed brings along extra nitrogen which absorbs a portion of the heat generated in the kiln, thus reducing the heat available for the process and, correspondingly, the temperature level resulting in reduced destruction efficiency of organics.
When a portion of the waste charged into the kiln during a certain time period has lower caloric value than the expected design value, the kiln temperature can decline due to reduced heat release. This may lead to the formation of cold spots in the furnace when local temperatures decrease below the ignition point for some organics. The result is a low temperature failure mode with a substantial breakthrough of the original organics which cannot be destroyed at lower temperatures. A drastic increase in PIC formation may also occur due to quenching of pyrolytic products formed from the original wastes and fuel.
Other failure modes may occur as a result of poor atomization of liquid wastes and poor mixing of wastes with available oxidizers. Poor atomization of liquid wastes leads to increased size of droplets resulting in incomplete combustion while poor mixing may provide an opportunity for the volatilized wastes to short circuit the combustion process, avoiding adequate contact with an oxidizer. Both of these failure modes result in products of incomplete combustion being transferred to the afterburner.
Flameout failure modes predominantly occur at unfavorable aerodynamic conditions in the combustion zone. High velocities of gaseous products near the burner during low fire conditions, a deficiency of oxidizer, and excessive infiltration of cold ambient air in the combustion apparatus are typically events which cause flameout. Excessive increase in the ambient air moisture content and the high moisture of the wastes being charged may be other sources of low temperature or flameout failure.
Failure modes similar to those described above for the kiln may also occur in the afterburner. In addition, overcharging, low residence time, low temperature, poor mixing, the cold wall effect, flameout and poor atomization in the kiln will always result in an increased PICs loading rate on the afterburner, and subsequently, in a lower thermal destruction efficiency overall for existing incineration systems.
Conventional incineration systems are hindered in their ability to address failure modes because the kiln, the afterburner, if used, and the air pollution control system are designed to operate in steady state conditions ignoring the existence of transient process disturbances which result in failure modes. Existing incineration systems are also unable to anticipate transient operational changes of the several individual elements of the incineration system. For example, they are not capable of rapidly boosting temperatures and oxygen content in the afterburner to overcome failure modes in the primary combustion apparatus.
Several attempts have been made to improve thermal destruction efficiency by enriching combustion air in the primary incineration means with oxygen (see, for example, U.S. Pat. Nos. 4,520,746; 4,462,318 and 4,279,208). The advantage of oxygen use in incineration processes is based on the reduction in the volume of nitrogen introduced into the incineration process. This reduction in the volume of nitrogen decreases the amount of heat stored in the nitrogen molecules making additional heat available for waste destruction and for increasing the temperature in the kiln. In addition, the use of oxygen reduces the quantity of gases flowing through the kiln, thereby increasing the residence time and the efficiency of destruction of persistent organics.
The use of oxygen in the waste incineration processes helps to stabilize combustion and to eliminate the possibility of failures related to low temperature, insufficient residence time and the negative impacts of low caloric wastes. However, the steady flow of additional oxygen may be only marginally effective in cases of transient overcharging, poor atomization and poor mixing, which are the failure modes most prone to the breakthrough of POHCs and formation of PICs. Permanently maintaining an elevated oxygen feed rate can result in overheating of primary incineration combustion apparatus and in damage to the metal parts and refractories. Moreover, an increased oxygen feed results in added operational costs. Although the additional use of a permanent oxygen flow may improve the destruction efficiency of kilns and afterburners, it cannot solve the problems related to the transient changes such as those caused by batch charging, poor atomization and poor mixing. This also cannot help to optimize the destruction efficiency at a given capacity or to maximize the capacity of the facility at a given or required efficiency. Existing methods cannot reconcile the conflict among the desired factors of high temperature, retention time, turbulence, and oxygen level in furnaces.
There exists, therefore, a need for an incineration system and method which results in the efficient destruction of liquid and solid wastes.
Further, there exists a need for a system and method which solves the problems related to the transient changes such as those caused by batch charging, poor atomization and poor mixing.
Also, there exists a need for a system and method capable of identifying critical prefailure conditions of the process and providing optimum levels of fuel, oxygen and air to be fed into the system.